Damage
Damage is estimated.
A small item such as a dagger wouldn't deal much damage if it hit a toe for instance, but if it gouged someone's eye out, it may well remove the eye and if the dagger was long enough, kill them. Also, a warhammer may be likely to deal lots of damage no matter where it hits as it may crush the person's bones.
Defending
Be reasonable. A block with the arms wouldn't help much against a warhammer, but if they were to intercept a dagger or a stone, they may reduce the damage. You may still be left with bruises though.
If you were to block with a shield and were up against a warhammer, the vibrations may shatter bones in your arm but a dagger may easily be blocked.